1. Field of the Invention
This invention relates to a vacuum interrupter for use in a high voltage circuit. More particularly, it relates to a vacuum interrupter including an evacuated envelope equipped with pressure monitoring means having a vacuum pressure detector element for monitoring the internal pressure of the evacuated envelope at all times and which applies to the vacuum pressure detector element a voltage obtained by dividing the voltage of the high voltage circuit as applied to the vacuum interrupter.
2. Description of the Prior Art
A vacuum interrupter comprises an envelope held at a high vacuum and a pair of relatively movable electrodes located within the envelope. When the interrupter is in the closed position, the pair of electrodes contact each other to conduct a current. The interrupting operation is effected by separating the electrodes. An arc produced between the electrodes at the time of separation is extinguished by employing an arc diffusing action in the vacuum and is prevented from reigniting by the high dielectric strength characteristic of the high vacuum.
In order to extinguish the arc and prevent it from reigniting, it is important to maintain the pressure inside the envelope below a predetermined value. Accordingly, some means must be provided for the vacuum interrupter to monitor the internal pressure of the envelope. Means for monitoring the vacuum pressure inside the envelope of the vacuum interrupter at all times is disclosed in, for example, the specification of U.S. Pat. No. 3,403,297.
This pressure monitoring means for the vacuum interrupter is so constructed that, when the vacuum interrupter is in the closed state, a voltage is applied from a separate power source between an intermediate metal shield arranged within the envelope and either electrode of a pair of separable electrodes for interrupting the current. More specifically, when the pressure inside the envelope rises, the generation of a glow discharge due to the voltage applied between the electrodes is detected to issue an alarm. Besides, in order to avoid any damage of the vacuum interrupter, a current-responsive detector is operated by the glow discharge current so as to prevent the vacuum interrupter from being opened.
On the other hand, when the pressure inside the envelope is normal, the intermediate metal shield is held at a floating potential in order not to interfere with or undesirably affect the interrupting operation. Therefore, the voltage applied from the separate power source is removed prior to the start of the interrupting operation, and the monitoring means is electrically detached. Thus, the pressure monitoring means for the vacuum interrupter as described above is undesirable in that it requires a separate power source, and has a complicated circuit arrangement for detaching the power source prior to the start of the interrupting operation of the vacuum interrupter.
Another example of a method of pressure monitoring means in a vacuum interrupter of this type is one known as the magnetron method. According to this pressure monitoring method, a pair of pressure detector electrodes are insulated and constitute a vacuum pressure detector element which communicates with the envelope of the vacuum interrupter. A magnetic field is applied in the area between the electrodes, while an electric field crossing the magnetic field is provided. Thus, any residual gas inside the envelope is ionized, and an ionic current flows between the pair of electrodes. The ionic current is measured to indicate the residual gas pressure. Since, however, such a vacuum interrupter is connected to the high voltage circuit, the power source for applying the voltage between the electrodes of the vacuum pressure detector element must be electrically insulated by the use of, for example, an insulating transformer.